I use multiple usb's during install 1- external usb hdd to install to & my Cruzer for installlation media.

I think the OP should choose Advanced Options>> Expert Install so that they are presenetd with more options.
Theonly real option thatI would worry about would be placement of bootloader, which I know that the option to install it to sdb or sdc would be there.

What is difference between Linux packages on CD(650MB) and DVD(4.4GB)? Is there a preference between them? Will the installer consider SSH drive as installation drive straight after boot, or I have to select it? Where to read the description of Graphic installer and how to set option for custom install?

Packages on cd are minimalistic although you can get anything that's available on cd as on dvd as long you have an internet connection. In fact if you have an internet connection you can install with the mini iso as you'll get the updates during installation.

its not automatically placed in proper place by installer?

What proper place? Everyone doesn't have the same setup and the developers don't assume that their distribution is the one and only so users are given options. You can install to the master boot record, the partition of the root filesystem, a separate boot partition and mbr, and EFI partition or in some cases, not install at all.

So one form of normal install and normal situation would be that your PC would try to boot off of media in the CD/DVD drive so that if you did have a restore disk in there, it would boot off of that. Many newer PCs and especially laptops don't use restore CDs and instead they have a special partition on the internal hard disk and so then they'd have a special hot key at startup that would show you boot options, as jefro has pointed out. This is all why I recommended earlier that you research the startup and boot options specific to your PC. That said, this is either a fully constructed PC as bought, or something which has been assembled by your or someone else. But still, it has BIOS, you should be able to get into SETUP, and from your replies indicate that you have managed that.

Sounds also like you have a DVD burned from the ISO file.

Therefore you can set your boot options to boot off of CD/DVD and it should boot the version of Linux you have on that DVD. This also will not install without your consent. In fact, many of them will load Linux fully and then give you an option to just play with Linux or actually install it. What I mean by load Linux is that it will boot, it won't modify your hard drive, it will basically be running off of the DVD.

When you choose to install Linux, it will search your system for available, writable disk drives and then give you the option to choose where to install Linux. The issue there is that it will not show you letters exactly like you see in Windows. For instance your internal hard drive in Windows is likely C: and if you attach an external USB hard drive, it may label that one as E: or F:. Those letters will be different for Linux and also shown differently; Linux will use designations like /dev/sda, /dev/sdb, and so forth.

All I can tell you at that point is that a good installation process will show you the drive options and also advise you how much space is available on them for your installation. For instance, say your internal hard drive is 500G and Windows takes up 250G of that, so it should show you a 500G hard drive with about 250G free. And say your external SSD drive is 1 Terrabyte, or 1000G, it should show you a 1TB hard drive with almost all of it free (assuming you have pretty much nothing on that drive at the time). It MAY show you other drive partitions; for instance say it labels your internal hard drive as /dev/sda, then maybe it will say /dev/sda1 is 490G with 240G free and then says there is a /dev/sda2 partition of size 10G with 1G free. That perhaps would be a backup/recovery partition. These numbers are by no means exact or even close, I'm just citing an example. And further to note would be that the "letter" of each physical drive should be different; therefore if your internal hard drive is /dev/sda, then the external SSD drive would another letter, such as /dev/sdb, or /dev/sdc.

So you can boot Linux to see how it runs on your system at no changes to your system. You can also install it at any time of your choosing and the install procedure will assist you in determining the target drive to install too, but you have to use a little bit of smarts when you do that, by way of paying attention to what it tells you and being cognizant of the options it allows you to use.

As far as boot, I believe if you say that you wish to install all of Linux onto that external SSD drive then it will work, I do believe that minor modifications will be made to your internal hard drive to change how the system boots, but with the intentions of doing no harm to your data on that drive.

After installing Linux the boot order should be changed back to default I assume?

Hi,

If you altered in the BIOS then you should revert to the previous setting.

Hope this helps.
Have fun & enjoy!

I want to reserve space for some other linux distro in the future, how should be specified that partition?

A 60Gig drive isn't very large you can get a few OS on it but you'll find that the OS you have the less the space for data.

Does it make sense to leave some unallocated space in ssd?

It really is up to you. While an ssd is not a mechanical drive. Bits are still bits. If you want you can reserve what you need or what.

For a home user 16g is about what you'd want to have for a distro. At about 8 gig things get tight. More than 16 gig would be for many many apps or lots of data or multiple window managers.

You can always have some common place for downloads and stored media. Those should be on a mechanical drive actually.

I burned an ISO image to DVD with ImgBurn use 'Write' mode, the burned image contains the entire contents of the ISO disk image(expanded).

I've used Imgburn in the past and am sure it has an option to 'burn as an image' which should make the CD/DVD bootable. If you open the CD/DVD from a Linux or windows system and see a number of directories/files then you did it right. If you just see the iso file, you did it wrong. From your last post, it seems you did it correctly. Does it boot? What's your question?

yes, it is. Does it require to set a separate boot partition? Should I specify all partitions as primary? How to create partition for a potential another Linux distribution?